Method for preparing magnetic compositions



United States Patent Ofiice 2,705,701 Patented Apr. 5, 1955 METHOD FOR PREPARING COMPOSITIONS Henry L. Crowley, South mesne assignments, Inc., West Orange,

MAGNETIC Orange, N. J., assignor, by to Henry L. Crowley & Company, N. J., a corporation of New Jersey No Drawing. Application November 28, 1952, Serial No. 323,145

9 Claims. (Cl. 252-625) This invention relates to magnetic compositions and therefrom, and has for its object the pl'OVlSlOIl of certain improvements in methods of making More particularly, the invention contemplates by Weight of ferric oxide and one or more other metal oxides is subjected to multi-stage heattreatment including prefiring and final-firing at temperatures Within the range of 800 to 1500 C. The invention more specifically involves certain improvements in the preparation of the product of the pre-firing or first stage heat-treatment for the last or final stage of heat-treatment Magnetic compositions, commonly known as ferrites, are prepared by subjecting to two or the environments in which the the pre-fired product are respectively heat-treated are so controlled that ferrous oxide is present in the product resulting at the conclusion of final-firing, and that product is gradually cooled through at least the greater part of the temperature range in which ferrous oxide spontaneously dissociates into metallic iron and of the final article (e, g, a magnetic core), due allowance being made for shrinkage during the second heat-treatment. The shaped articles are then subjected to final-firing, and, when made in accordance with the method of patent 2,575,099, are gradually cooled as hereinbefore mentioned.

I have discovered that the electrical, magnetic and usefully enhanced, as well as beneficially controlled, by selectively sizing the finely ground product of pre-firing to give the greatest possible packing density in compactfiring, as described in my Patent No. 2,703,787, and the present 1nven t1on, in its preferred aspect, accordingly involves select1ve sizing of the finely-ground product of microns, about one-quarter (or slightly more) by Weight is of a particle size between 10 the balance is of a particle size up to 5 microns and preferably between 1 to 5 microns.

powder is formed into densely-packed shaped artlcles and a the shaped articles are subjected to final-firing in any of the aforementioned methods of positions.

In practicing the present invention in conjunction with the invention of my Patent No. 2,703,787, the initial oxide mixture siderable mg. Prior to compacting, the initial oxide mixture is subected to a relatively high vacuum, sufiicient to substantially deaerate the mixture, so that the as practical, mercury, and and even higher if practical.

In compacting and densifying the deaerated initial oxide lxture,

by weight of the initial oxide mixture. (Water) content of about 10% temperature slightly above and are then pre-fired.

At the conclusion of the pre-firing or first-stage heattreatment, particularly when conducted in a reducing environment, the heat-treated shapes are preferably quenched tively fine powder.

Grinding is carried out to enable selective sizing of the ground product in accordance with the characteristic principles of the present invention. The aim of such selective sizing is to produce a powder in which the relatively coarse and relatively fine particles are so proportioned as to give shaped articles for final firing is as follows: Particle size in microns:

Percent by weight 35-25 -10 40-50 -15 -20 25 -40 The by weight of preferably made up of the 10 to micron portion is about three equal amounts by weight of the 10-15 micron, 15-20 micron and 20-40 micron portions, respectively. Thus, only about 8% by weight of the finely ground and selectively-sized powder is of a particle size exceeding 20 microns, and only about 17% is of a particle size exceeding 15 microns. When the binder (customarily mixed with the powder for compacting and pressing into shaped articles) contains a waxlike lubricant as hereinafter mentioned, a larger proportion of the coarser particles may be tolerated, say 20% of the 10-15 micron particles and 20% of the 1540 micron particles, with 40% of the 5-10 micron particles and 20% of the finer particles up to 5 microns and preferably between 1 and 5 microns.

In practicing the invention, the pro-fired product is ground to give as near as practical a powder having this characteristic distribution of particle size. Since this re quires very careful grinding control, and even then is difiicult of attainment, an alternative practical and convenient procedure is to blend powders of known particle size ranges. For example, one portion (A) of the prefired product is ground to a size as uniform as possible to produce a ground product of which by weight is of a particle size between 2 and 6 microns. Another portion (B) of the pre-fired product is ground, as uniformly as possible, to produce a ground product having an average particle size of 25 microns, or alternatively until at least by weight of the ground product is of a particle size between 10 and 40 microns. 75% by weight of ground product A is then blended with 25% by weight of ground product B to produce a blended powder of the contemplated particle size distribution having excellent packing density.

Particle size determinations may conveniently be made with a Cenco-Sheard-Sanford Photelometer, obtained from the Central Scientific Company, of Chicago. The determinations are made by allowing a portion of the ground product to settle through a transparent liquid of known viscosity and depth. A beam of light is passed through the liquid and a photoelectric cell arrangement records the amount of light passed in a given time. As the particles settle through the liquid they interrupt the light. Since the particles settle in accordance with Stokes law, the amount of light that is cut off during a given time period is correlated to the percentage of particles of a certain particle size or within a-particular range of particle size.

In plant practice it is customary to make a particle size determination of a representative sample from each grind of the pre-fired product. Where this determination shows that the ground product does not meet the required particle size distribution, the product may be further ground, or it may be set aside for blending. By properly blending different grinds of known particle size distribution, a powder of the characteristic particle size distribution of the invention is readily obtained.

The powder, selectively-sized in accordance with the invention, is compacted and pressed into shaped articles with the aid of an appropriate binder. I have found that the binder should be composed of three parts, one for imparting a lubricating quality during pressing, the secnnd for imparting initial moist plasticity, and the third for imparting green strength to the shaped articles. The lubricating part is preferably a wax-like material, and may advantageously be compounded by heating a mixture of 6 parts by weight of stearic acid and 4 parts of triethanolamine to approximately 190 F. until a clear melted liquid (straw color) is obtained, and while held at 190 F. adding 15 parts of tricresyl phosphate. During cooling of the wax-like reaction product, sufficient water is added so that when the complete binder is mixed with the powder a moist, plastic mass is obtained. The water constitutes the second part of the binder, and the amount required is generally of the order of 10% on the weight of the powder with which the complete binder is to be mixed. To the cooled mixture of wax-like reaction product and water is added 4 to 8 parts by weight (calculated on a dry basis) of an organicresin, such for example as a phenol formaldehyde resin. The resin is preferably added in liquid form, as an aqueous solution when watersoluble or in an organic solvent such as an alcohol, a ketone etc. About 2 pounds (on a dry basis) of the complete binder is mixed with each pounds of the selectively-sized powder, mixing being conveniently carried out in a dough-type mixer. The resulting mixture may advantageously be dried in the mixer, or in an oven at a temperature of about 220 F., to eliminate the water. The dried material is easily granulated and flows readily into the press cavities. The fluidity needed for pressing is maintained since the residual plasticizer and resin do not vaporize at the drying temperature. In final-firing the non-resinous part of the binder vaporizes easily and rapidly within the temperature range of 400 to 600 F. The resinous part of the binder imparts green strength, both as pressed and while heating up to relatively low temperatures, and also supplies the carbonaceous reducing agent contemplated during final-firing in the method of my Patent 2,575,099. The carbonaceous residue should be kept to the required minimum, since more than required for the contemplated reduction is diflicult to burn out. Hence, the amount of resin (on a dry basis) mixed with the selectively-sized powder (in the form of the aforementioned complete binder) is about 0.25% to about 0.5% by weight.

Shaped articles produced in accordance with the ciples of the invention possess superior mechanical and physical properties. This is in large part due to the high packing density of the selectively-sized powder of which the shaped articles are formed. Additionally such shaped articles have improved shrinking properties during finalfiring. Moreover, practice of the invention, especially in its preferred and complete aspect, results in far greater uniformity of the finally-fired shaped articles. Thus, by the practice of the invention, rejects are reduced from around 50%, heretofore common in plant practice, to around 2%, and often to none, and usually the reasons for rejection are chips and minor physical imperfections occasioned in the course of ordinary handling. The invention also makes possible more advantageous and more predictable control of the physical and electrical properties of the finally-fired shaped articles. It has been found in practice that the electrical properties of the finished articles are greatly influenced by the soundness of structure, and when this soundness of structure is uniformly controlled, the electrical properties are more uniform and more predictable. The invention permits very accurate control of shrinkage during final firing. Not only can the shrinkage be substantially less than heretofore common in plant practice, but it can be controlled to meet various requirements.

Because of the influence that shrinkage exerts on the physical and electrical properties of the finished article, the ability to control shrinkage is of particular importance and advantage in plant practice. Not only is such control conducive to better uniformity of product, but it enables desirable adjustments in certain processing procedures and the attainment of optimum shrinkage for particular articles. For example, pressures of the order of 25,000 pounds per square inch have heretofore been customary in forming shaped articles for final-firing, and shrinkage during final-fiiring is about 15%. Formed under the same pressure, shaped articles made in accordance with the present invention may shrink during final-firing less than 5%, and even as little as l to 2%. Alternatively, where a predetermined shrinkage during final-firing is desired. a very considerably lower forming pressure may be employed with selectively-sized powders of the invention. Thus, using existing dies designed for a shrinkage of 15% during final firing, a forming pressure of only 5,000 pounds per square inch is required with selectivelysized powders of the invention, as contrasted with the heretofore customary forming pressure of 25,000 pounds per square inch.

Some magnetic articles are made of two or more sepaprin- 6 rate pieces, such, for example, as the conventional E- particle size larger than 10 microns but not larger than clamp in which two pieces, each of E-shape, are fitted about 40 microns, and the balance is of a particle size together to form a closed magnetic core. The electrical smaller than microns but not smaller than about 1 miproperties of the composite article are determined by cron, forming said selectively-sized powder into denselyties of the composite article. Accordingly, in making in which an initial oxide mixture containing at least 50% separate pieces which are to be fitted together to form by weight of ferric oxide and at least one other metal a composite article, a controlling consideration is the oxide is subjected to heat-treatment in multi-stages intightness of the fit that can be obtained. If shrinkage eluding pre-firing and final-firing at temperatures within during final-firing of such pieces is of the order of 10 the range of 800 to 1500 C., the improvement which to even small variations in shrinkage control will comprises grinding the pre-fired product to a relatively result in objectionable lack of mechanical uniformity 15 fine powder, preparing from the finely-ground pre-fired with attendant serious impairment of the electrical propproduct a selectively-sized powder characterized by the erties of the composite article. Even if such pieces do following particle size distribution 3525% by weight possess intrinsically good electrical properties, the elecof a partlcle size between 1 and 5 microns, 40 50% b trical properties of the composite article are likely to be weight of a particle size between 5 and 10 microns, 5%

poor because the chances of obtaining an unsatisfactory by weight of a particle size between 10 and 40 microns,

best results are obtained in making such composite articles shaped articles, and sub ecting said articles to final-firing y controlling shrinkage to a low degree, say 1 to 2%, at a temperature within the aforementloned range with some slight sacrifice 1n the intrinsic electrical proper- 6 In a method of preparing a magnetic composition acties, and practice of the present invention makes this p0scording to cla im further characterized in thatthe parin makin articles some shrinkage is not only unob ectionab e but between 10 and 15 microns, (2) between 15 and 20 beneficially effects the electrical properties of the article. microns, and (3) between 20 and 40 microns. The present invention by promoting control of shrinkage 7. In a method of preparing a magnetic composition and uniformity of product, facilitates processing and finalin which an initial oxide mixture containing at least 50% firing of such articles to give the optimum shrinkage, say by weight of ferric oxide and at least one other metal around 15%, for the best electrical properties. oxide is subjected to heat-treatment in multi-stages in- I claim: eluding prefiring and final-firing at temperatures within 1. In a method of preparing a magnetic composition the range of 800 to 1500 C., the improvement which in which an initial oxide mixture containing at least 50% comprises compacting the intimately mixed initial oxide by weight of ferric oxide and at least one other metal mixture into dense shapes under a pressure of at least oxide is subjected to heat-treatment in multi-stages in- 5,000 pounds per square inch while maintained under a eluding pre-firing and final-firing at temperatures within relatively high vacuum, subjecting the compacted dense the range of 800 to 1500 C., the improvement which shapes to pre-firing at a temperature within the aforecomprises grinding the pre-fired product to a relatively mentioned range, grinding the resulting pre-fired product fine powder, preparing from the finely-ground pro-fired to a relatively fine powder, prior to final-firing preparing product a selectively-sized powder of which from about from the finely-ground pre-fired product a selectively-sized 40% to about 50% by weight is of a particle size bepowder of which from about 40% to about by weight tween 5 and 10 microns, from about 35% to about 20% is of a particle size between 5 and 10 microns, from about by weight is of a particle size up to 5 microns, and from 35% to about 20% by weight is of a particle size up to 5 about 40% to about 25% by weight is of a particle size microns and from about 40% to about 25 by weight is between 10 and 40 microns, forming said selectively-sized of a particle size between 10 and 40 microns, forming said powder into densely-packed shaped articles, and sub- 50 selectively-sized powder into densely-packed shaped arjecting said articles to final-firing at a temperature within ticles, and subjecting said articles to final-firing at a the aforementioned range. temperature within the aforementioned range.

In a method of preparing a magnetic composition 8. In a method of preparing a magnetic composition according to claim 1 in which the selectively-sized powder according to claim 7 in which the selectively-sized powder is made up of about 40% by weight of particle sizes 5 is made up of about 40% by weight of particle sizes be between 5 and 10 microns, about 40% by weight of tween 5 and 10 microns, about 40% by weight of particle particle sizes between 10 and 40 microns and about sizes between 10 and 40 microns and about 20% by 20% by weight of particle sizes up to 5 microns. weight of particle sizes up to 5 microns.

3. In a method of preparing a magnetic composition 9. In a method of preparing a magnetic composition according to claim 2 in which about half of the 40% according to claim 8 in which about half of the 40% porportion of particle sizes between 10 and 40 microns is of tion of particle sizes between 10 and 40 microns is of a a particle size between 10 and 15 microns. particle size between 10 and 15 microns.

n a method of preparing a magnetic composltron Ln whichhan 1initial oxide mixture containing at least 50% References Cited in the file of this patent y weig t o ferric oxide and at least one other metal oxide is subjected to heat-treatment in multi-stages in- UNITED STATES PATENTS eluding pre-firing and finalring at temperatures within 2,283,925 Harvey May 26, 1942 the range of 8(00 toh1500fiC., the iimprovement which 2,575,099 Crowley Nov. 13, 1951 comprises grin ing t e prered pro uct to a relatively fine powder, preparing from the finely-ground pre-fired OTHER REFERENCES product a selectively-sized powder of which approxi- Goetzel, Treatise on Powder Metallurgy, Interscience mately one-half by weight is of a particle size between Pub. Inc., New York (1949), vol. 1, pp. 83, .87, 103-405, 5 and 10 microns, about one-quarter by Weight is of a 124 (Copy in Div. 3.) 

1. IN A METHOD OF PREPARING A MAGNETIC COMPOSITION IN WHICH AN INITIAL OXIDE MIXTURE CONTAINING AT LEAST 50% BY WEIGHT OF FERRIC OXIDE AND AT LEAST ONE OTHER METAL OXIDE IS SUBJECTED TO HEAT-TREATMENT IN MULTI-STAGES INCLUDING PRE-FIRING AND FINAL-FIRING AT TEMPERATURES WITHIN THE RANGE OF 800 TO 1500* C., THE IMPROVEMENT WHICH COMPRISES GRINDING THE PRE-FIRED PRODUCT TO A RELATIVELY FINE POWDER, PREPARING FROM THE FINELY-GROUND PRE-FIRED PRODUCT A SELECTIVELY-SIZED POWDER OF WHICH FROM ABOUT 40% TO ABOUT 50% BY WEIGHT IS OF A PARTICLE SIZE BETWEEN 5 AND 10 MICRONS, FROM ABOUT 35% TO ABOUT 20% BY WEIGHT IS OF A PARTICLE SIZE UP TO 5 MICRONS, AND FROM ABOUT 40% TO ABOUT 25% BY WEIGHT IS OF A PARTICLE SIZE BETWEEN 10 AND 40 MICRONS, FORMING SAID SELECTIVELY-SIZED POWDER INTO DENSELY-PACKED SHAPED ARTICLES, AND SUBJECTING SAID ARTICLES TO FINAL-FIRING AT A TEMPERATURE WITHIN THE AFOREMENTIONED RANGE. 